I think that most English speakers would interpret "He is a little bilingual" as "he is a little bit bilingual" - in other words, "bilingual" is an adjective, and you would not use the copula to say that, - Tá an fear beagán dátheangach is OK.
If you wanted to interpret "bilingual" as a noun, and use "little" as an adjective ("small") to describe the bilngual's stature, then you probably wouldn't use dátheangach in the first place, because dátheangach is an adjectival form, and dátheangeolaí might be a better choice, but if you did use dátheangach as a noun, then you would say Is dátheangach beag é an fear.
Go raibh maith agat! I was grappling with this. It's always a case of getting caught on little things like this and getting discouraged, then learning it one way or another, and getting back into it with a vengeance. My future as a linguist doesn't seem that paper-thin, looking at how much I enjoy this. :D
Through Duo I learned it like you said, but what about the sentence "Is fíor é"? There the predicative is also just an adjective, but Duo uses "is" instead of "tá". I also remember learning from the book "Teach Yourself Irish" that nationalities are stated with "is", so is it impossible to say "Is dátheangach an fear beag é", and if, is there a reason why?
You could say Is duine dátheangach é an fear beag. Here is what Gearrchúrsa Gramadaí says about the copula:
Ceanglaíonn an chopail is dhá fhocal nó dhá abairtín dá chéile chun a thaispeáint gurb ionann iad. Tugann an chéad chuid den abairt (an fhaisnéis) eolas faoin dara cuid di (an t-ainmní).
The prefix for 'multi' is il- which lenites except before d, s, t. So that would give il-teangach.